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Sonocatalytic removal of ibuprofen and sulfamethoxazole in the presence of different fly ash sources
Ultrasonics - Sonochemistry 39 (2017) 354–362
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Sonocatalytic removal of ibuprofen and sulfamethoxazole in the presence of different fly ash sources
MARK
Yasir A.J. Al-Hamadania, Chang Min Parka, Lateef N. Assia, Kyoung Hoon Chua, Shamia Hoquea, ⁎ Min Jangb, Yeomin Yoona, , Paul Ziehla a b
Department of Civil and Environmental Engineering, University of South Carolina, Columbia, SC 29208, USA Department of Environmental Engineering, Kwangwoon University, 20 Kwangwoon-ro, Nowon-Gu, Seoul 01897, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Ibuprofen Sulfamethoxazole Sonocatalytical degradation Fly ash Water treatment
We examined the feasibility of using two types of fly ash (an industrial waste from thermal power plants) as a low-cost catalyst to enhance the ultrasonic (US) degradation of ibuprofen (IBP) and sulfamethoxazole (SMX). Two fly ashes, Belews Creek fly ash (BFA), from a power station in North Carolina, and Wateree Station fly ash (WFA), from a power station in South Carolina, were used. The results showed that > 99% removal of IBP and SMX was achieved within 30 and 60 min of sonication, respectively, at 580 kHz and pH 3.5. Furthermore, the removal of IBP and SMX achieved, in terms of frequency, was in the order 580 kHz > 1000 kHz > 28 kHz, and in terms of pH, was in the order of pH 3.5 > pH 7 > pH 9.5. WFA showed significant enhancement in the removal of IBP and SMX, which reached > 99% removal within 20 and 50 min, respectively, at 580 kHz and pH 3.5. This was presumably because WFA contains more silicon dioxide than BFA, which can enhance the formation of OH% radicals during sonication. Additionally, WFA has finer particles than BFA, which can increase the adsorption capacity in removing IBP and SMX. The sonocatalytic degradation of IBP and SMX fitted pseudo first-order rate kinetics and the synergistic indices of all the reactions were determined to compare the efficiency of the fly ashes. Overall, the findings have showed that WFA combined with US has potential for treating organic pollutants, such as IBP and SMX, in water and wastewater.
1. Introduction Over the last two decades, large quantities of products, such as medicines, disinfectants, and personal care products, have been released into surface waters and wastewater treatment facilities by the pharmaceutical and chemical industries [1]. Increases in the concentrations of some pharmaceutical compounds, such as ibuprofen (IBP) and sulfamethoxazole (SMX), have come to the attention of scientists with regard to their impacts on life in lakes, rivers, and groundwater [2,3]. Irregular disposal of unused medications, expired drugs, and veterinary medicines are the majors reasons why they end up in water bodies [4,5]. Concentrations of IBP and SMX in surface waters have been detected in the range of 30–480 ng L−1 [6,7], creating unique challenges, as conventional water treatment processes, including coagulation/sedimentation/filtration, typically can only remove 10–20% of these compounds [8,9]. Consequently, efforts are needed to find effective processes to remove these contaminants from water to meet the important goal of providing safe drinking water. Ultrasonic (US) treatment is one of the promising advanced oxida-
⁎
Corresponding author. E-mail address: [email protected] (Y. Yoon).
http://dx.doi.org/10.1016/j.ultsonch.2017.05.003 Received 4 January 2017; Received in revised form 18 April 2017; Accepted 2 May 2017 Available online 03 May 2017 1350-4177/ © 2017 Elsevier B.V. All rights reserved.
tion processes that has the potential to produce hydroxyl radicals (OH%) in water, which are strong oxidizing agents [10]. The power of OH% in water treatment lies in their ability to destroy and degrade complex, otherwise-hard-to-degrade, and toxic organic compounds and convert them ultimately to carbon dioxide and water [11]. The process of US treatment produces OH% through the cavitation phenomenon and the formation of high-intensity bubbles [12]. Cavitation occurs very quickly, through the steps of nucleation, growth, and the collapse of cavitation bubbles in water, releasing large amounts of energy locally, generating hot spots, and producing hydrogen and OH% due to the sonolysis of water [5,10]. During this phenomenon, high temperatures (5000 K) and pressures (1000 atm) created inside cavitation bubbles lead to thermal dissociation of water molecules into H% and OH% [13]. The O2 dissolved in water reacts and forms OH% and HO2%. Additionally, the cavitation bubbles contain three zones: the gaseous zone, the gas–liquid transition zone, and the bulk liquid zone. In the gaseous zone, the temperature and pressure reach their maximum levels of 5000 K and 1000 atm, respectively. The zone is hydrophobic and volatile compounds can be degraded. Second, in the gas–liquid
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and efficient in producing a consistent, high-quality fly ash [26]. Stock solutions of IBP, SMX, BFA, and WFA were prepared in ultrapure deionized (DI) water.
transition zone, the temperature reaches 2000 K. The zone is moderately hydrophobic and moderate degradation of volatile compounds can be achieved. The third zone is the bulk liquid zone, where the temperature is 300 K; hydrophilic and non-volatile compounds undergo degradation in this zone [14–16]. Previous studies have indicated that US treatment has marked benefits, including safety, cleanliness, and ease of use; additionally, no carcinogenic by-products form during treatment [5,11]. Many studies have reported that sonochemical degradation can be enhanced by the presence of solid surfaces as catalysts, such as TiO2, quartz, glass beads, polyaluminum chloride, Al2O3, and carbon nanotubes [5,15,17,18]. Because US treatment is highly energy-intensive, catalysts are needed to improve the removal efficiency and reduce the effective energy consumption [17]. However, such catalysts are relatively expensive for treating large volumes of wastewater. Thus, alternative low-cost catalysts need to be investigated for their ability to remove contaminants, such as pharmaceutical compounds. Fly ash was assessed in this study regarding its ability to enhance the sonodegradation of IBP and SMX. Fly ash is a by-product waste material generated in dry form in thermal power plants. Large amounts of fly ash are generated and dumped in landfills annually [19–21]. Thus, using fly ash in water and wastewater treatment is a good strategy to reduce environmental pollution. Fly ash’s chemical composition generally consists of aluminum oxide (Al2O3) and silicon dioxide (SiO2) (in total, 60–80 wt%), in addition to some transition metal oxides [13,19]. Previous studies have found fly ash to be a good adsorbent for various types of dyes [22,23], and it has been used in photocatalytic applications, combined with TiO2 [24]. However, only few studies have investigated the use of fly ash as a catalyst under ultrasonic irradiation, despite its ability to enhance significantly the sonodegradation of acid orange 7 [13]. This suggests that fly ash may have the potential to enhance the removal of pharmaceutical compounds, such as IBP and SMX, under different frequency and pH conditions. Thus, the objectives of this study were to evaluate the removal of IBP and SMX at different US frequency and pH conditions in the presence and absence of fly ash. Reactions were carried out as a function of frequency (28, 580, and 1000 kHz) and pH (3.5, 7, and 9.5) at a fixed power (0.18 W mL−1) and room temperature (15 °C). The contribution of this work was to investigate an alternative low-cost catalyst (fly ash) that may enhance the removal of IBP and SMX. Two hypotheses were tested. First, fly ash should enhance the removal of IBP and SMX, due to the increased production of OH% radicals (a strong oxidant). This is presumably because fly ash contains sufficient amounts of Al2O3 and SiO2 that can react with the hydrogen peroxide and generate OH% radicals. Second, US irradiation should enhance the adsorption activities of fly ash due to the dispersion resulting from the harsh conditions provided by US irradiation. This would be expected to lead to an increase in the adsorption sites on fly ash particles.
2.2. Apparatus The US process was performed in a double-jacketed stainless steel reactor (L × W × H: 15 × 10 × 20 cm) with a water-cooled (Fisher Scientific Inc., Pittsburgh, PA, USA) US generator (Ultech, Dalseo, Daegu, South Korea). The sonicator provided three test frequencies: 28, 580, and 1000 kHz. The applied power in all tests was 0.18 W mL−1. The US applied power was measured by calorimetry by thermocouple (HI 9063, Hanna Instruments Ltd., Leighton, UK). Fig. S1 shows a diagram of the experimental set-up. Because BFA and WFA were used as catalysts, an optimum dose was determined based on H2O2 production at different fly ash dosages and frequencies. Adsorption experiments with the adsorbents (BFA and WFA) and adsorbates (IBP and SMX) were performed for 60 min, in a batch reactor with no US irradiation. BFA and WFA were hydrated for 24 h in DI water and stirred with a magnetic stirrer at 600 rpm prior to being added to the reactor vessel. The initial stock solution was 1000 mL, which was used in all experiments (US only, US with BFA/WFA, and the adsorption experiments). Samples were taken periodically and filtered through 0.22-μm glass microfiber filters to preserve uniformity and to eliminate larger fly ash particles that might interfere with the measurements. 2.3. Analysis IBP and SMX concentrations before and after treatment were measured using high-performance liquid chromatography (HPLC, 1200 series; Agilent Technologies, Santa Clara, CA, USA). The mobile phase was a 40%:60% mixture of deionized water:acetonitrile for IBP and a 50%:50% deionized water:acetonitrile for SMX. Separation was achieved with a LiChrosorb RP-18 analytical column (4.6 mm × 100 mm i.d., 5 µm particles, Atlantis; Waters, Milford, MA, USA) with a 100-µL sample loop at a flow rate of 1.0 mL min−1 for IBP and 0.75 mL min−1 for SMX. The wavelength used to detect the compounds was 210 nm. The KI dosimetry method was used to determine the H2O2 concentration, as an indicator of OH% free radicals in the system [27], using a 350-nm wavelength and an ultraviolet–visible (UV–Vis) spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). The elemental composition of BFA and WFA fly ash was determined by X-ray florescence (XRF) using fused bead analysis at the Holcim Inc. laboratory in Holly Hill, SC, USA. 3. Results and discussion 3.1. Dose optimization and characterization of BFA and WFA Determining the optimum dose of fly ash is important in comparing the efficiency of BFA and WFA under US irradiation. The optimum dose was determined based on the H2O2 produced under irradiation; as a rule, the amount of H2O2 increases with the amount of OH% radicals. Thus, H2O2 was measured as an indicator of OH% radicals in the system [28]. Previous studies showed that the presence of solid surfaces can improve the sonodegradation of contaminants by increasing the formation rate of cavitation bubbles; the presence of solid particle in solution provides a nucleation site due to surface roughness, leading to increased generation of OH% radicals in the system [5,29]. The optimum values of various doses (0, 5, 15, and 45 mgL−1) of BFA and WFA were investigated at three frequencies (28, 580, and 1000 kHz) at pH 7, as shown in Fig. 1. The results clearly indicated that the maximum production rate of H2O2 was achieved at 45 mg L−1 at all frequencies for both BFA and WFA, However, WFA showed higher H2O2 production than BFA due to differences in the chemical properties between the fly ashes.
2. Materials and methods 2.1. Chemicals and fly ashes Table S1 lists the characteristics of the selected target chemicals (IBP and SMX) from the SRC PhysProp database [25]. All chemicals were used as received with no further purification. High-purity IBP (C13H18O2, > 98%), SMX (C10H11N3O3S, > 98%), potassium hydrogen phthalate (C8H5KO4, 99.95%), potassium iodide (KI, 99%), and ammonium molybdate tetrahydrate (H24Mo7N6O24·4H2O) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Two fly ashes, Belews Creek fly ash (BFA) from a power station in North Carolina and Wateree Station fly ash (WFA) from a power station in South Carolina, were investigated. The main difference between the two sources is that the Wateree Station source was subjected to a proprietary carbon burn-out process. The carbon burn-out process has been shown to be effective 355
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Fig. 1. Effect of ultrasonic frequency on H2O2 production in the presence of BFA and WFA; (a) BFA at 28 kHz, (b) BFA at 580 kHz, (c) BFA at 1000 kHz, (d) WFA at 28 kHz, (e) WFA at 580 kHz, and (f) WFA at 1000 kHz at pH 7.
leading to enhanced oxidation activity as described in Eqs. (13) [34]:
Table 1 XRF chemical composition analysis of BFA and WFA. Compound
Chemical formula
BFA wt.%
WFA wt.%
Silicon dioxide Aluminum oxide Iron oxide Calcium oxide Magnesium oxide Sulfur trioxide Sodium dioxide Potassium oxide Total alkali Diphosphorus pentoxide Titanium dioxide Manganese oxide
SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2 O – P2O5 TiO2 MnO2
48.9 20.9 6.9 3.9 1.0 0.6 5.0 2.7 8.7 0.3 1.0 0.1
58.4 22.4 6.6 0.6 0.6 0.1 2.4 2.1 5.5 0.1 1.1 0.1
Fe 2+ + H2 O2 → Fe3+ + OH% + OH−
(1)
Fe3+ + H2 O2 → Fe−O2 H2+ + H+
(2)
Fe−O2 H2+ → Fe 2+ + OOH%
(3)
The decomposition of H2O2 in the presence of fly ash containing metal oxides can be described by the Haber-Weiss mechanism [35]. In the Harber-Weiss mechanism, the principal role of H2O2 is the oxidation of the metal surface, which leads to the formation of hydroxyl radicals as described in Eq. (4),
S + H2 O2 → S + + OH% + OH−
(4)
where S represents the uncharged metal surface [36]. Thus, Fe2O3 has the potential to produce more OH% radicals by reacting with the H2O2 formed, leading to enhanced sonodegradation of the compounds. The remaining constituents in the fly ashes were not considered due to their trace amounts and their presumably minor or negligible effects on sonodegradation. Taken together, our results showed that WFA contained more SiO2 than BFA (Table 1), which increased the production of OH% radicals. Because the fly ash consists of a wide range of particles sizes, it was important to define the particle size distributions of BFA and WFA (Fig. 2). WFA contained finer particles than BFA; the average particles size for BFA and WFA were 21.3 and 15.2 µm, respectively, indicating an average particle size difference of 29%. This could be responsible for the adsorption behavior of the fly ashes because smaller particles have
The elemental composition of BFA and WFA is summarized in Table 1. SiO2 was a major component in both fly ashes. SiO2 is also a main component of glass beads, which have been shown in previous studies to be effective in increasing the generation of OH% radicals in US processes [5,30,31]. The presence of Al2O3 can also increase OH% because (i) the oxide would bind OH% radicals and thereby decrease the formation of H2O2 and (ii) both SiO2 and Al2O3 can react with the H2O2 produced due to the sonication, and reproduce OH% radicals [32,33]. A third major constituent in the selected fly ashes was Fe2O3. Fe2O3 can also enhance the production of OH% radicals in an aqueous system by reaction with the H2O2 produced; this in turn allows OH% to reform, 356
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decrease followed a pseudo first-order kinetic law, and the removal of IBP was the highest at 580 kHz (> 99% at 30 min, > 99% at 55 min, and 95% at 60 min at pH 3.5, 7, and 9.5, respectively) followed by 1000 kHz (98%, 77%, and 62% at pH 3.5, 7, and 9.5, respectively) at 60 min, and the significantly lower removal was obtained at 28 kHz (56%, 33%, and 22% at pH 3.5, 7, and 9.5, respectively) at 60 min. SMX removal showed a similar trend where maximum removal was obtained at 580 kHz (> 99%, 90%, and 76% at pH 3.5, 7, and 9.5, respectively) at 60 min, followed by 1000 kHz (92%, 70%, and 47% at pH 3.5, 7, and 9.5, respectively) and 28 kHz (43%, 21%, and 19% at pH 3.5, 7, and 9.5, respectively) at 60 min. The main factors at the different frequencies were the number of cavitation bubbles, bubble size, the cavitation threshold, and the lifetime of the bubbles before collapsing [5,37]. At 28 kHz, fewer cavitation bubbles were produced, the growth period to collapse was high, and large bubbles formed [38]; thus, fewer OH% free radicals were produced, leading to less degradation of IBP and SMX, due to the reduction in the oxidation activity in the system. In contrast, at higher frequencies (580 and 1000 kHz) the number of cavitation bubbles increased and the lifetime of the bubbles to collapse decreased. Overall, the more number of cavitation bubbles lead to increase the numbers of OH% free radicals. Thus the chances of the recombining and forming H2O2 might be higher as well. However, at high frequency, the retarding effect and the free radicals tended to move quickly towards the bulk liquid [15,38], reducing the possibility of OH% recombination and formation of H2O2; this resulted in increased removal of IBP and SMX [39]. However, a lower degradation rate was achieved at
Fig. 2. Particle size distribution of BFA and WFA.
more surface area and adsorption capacity for IBP and SMX. 3.2. Effects of frequency and pH on IBP and SXM removal and H2O2 production In the sonodegradation process, there are two important factors, frequency and pH, along with the physicochemical properties of the compounds, which play major roles in the degradation of contaminants. Fig. 3 shows the degradation of IBP and SMX at the three frequencies and three pH conditions (pH 3.5, 7, and 9.5). The concentration
Fig. 3. Effect of ultrasonic frequency and pH on IBP and SMX removal; (a) IBP at pH 3.5, (b) IBP at pH 7,(c) IBP at pH 9.5, (d) SMX at pH 3.5, (e) SMX at pH 7, and (d) SMX at pH 9.5.
357
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0.79 and 0.5 g L−1, respectively) as well as different pKa values (4.52 and 5.81 for IBP and SMX, respectively), which have combined effects on sonodegradation activity. Therefore, IBP had a tendency to consume more OH% radicals than SMX from the system, leading to more degradation of IBP than SMX, resulting in less recombination of OH% radicals to form H2O2 [39]. The second important factor influencing sonodegradation is the pH. Three pH values (3.5, 7, and 9.5) were investigated with the three frequencies to better understand the sonodegradation of IBP and SMX at a fixed power (0.18 W mL−1). The removal of both IBP and SMX was highest under acidic conditions, pH 3.5, and it decreased at pH 7 and pH 9.5 (Fig. 3). The degradation of IBP and SMX decreased as pH increased because the physicochemical properties of the compounds would put them in their molecular forms when the pH was lower than the pKa values, while above the pKa values, they would be in ionic forms [14]. The higher degradation of IBP and SMX under acidic conditions and US is because IBP and SMX, in their molecular forms, tend to accumulate at the boundary of the cavitation bubbles where the concentration of OH% radicals is maximal. In contrast, in their ionic forms, when the pH is higher than their pKa values, the compounds tend to move towards the bulk zone where the OH% concentration is lower [45,46]. Thus, under acidic conditions, more degradation of IBP and SMX was achieved because the reaction between the OH% radicals and the IBP/SMX occurred in a zone with a higher density of OH% radicals at the boundary of the cavitation bubbles and less under alkaline conditions because the reaction occurred in the bulk liquid zone, where fewer OH% radicals are found. The production of H2O2 was assessed at the selected pH values. Clearly, Fig. 4 shows that the generation of H2O2 followed a trend opposite to that of the degradation of IBP and SMX, in which the maximum H2O2 was produced at pH 9.5, followed by pH 7 and pH 3.5. This is because at low pH, the OH% free radicals have a tendency to attach and react with IBP and SMX more than recombining to produce H2O2, and thus more degradation and less H2O2 were achieved than at higher pH conditions [14,39]. In addition, in the absence of IBP and SMX, the production of H2O2 followed the order of 580 kHz > 1000 kHz > 28 kHz and pH 9.5 > pH 7 > pH 3.5 as shown in Fig. S2. The maximum H2O2 generated was achieved at 580 kHz (170 µM at pH 3.5, 211 µM at pH 7, and 232 µM at pH 9.5) followed by 1000 kHz (63 µM at pH 3.5, 81 µM at pH 7, and 105 µM at pH 9.5) and lowest H2O2 production was obtained at 28 kHz (12 µM at pH 3.5, 17 µM at pH 7, and 21 µM at pH 9.5).
Fig. 4. Effect of ultrasonic frequency and pH on H2O2 production in the presence of IBP and SMX at (a) pH 3.5, (b) pH 7, and (c) pH 9.5.
3.3. Effects of frequency and pH on IBP and SMX removal in the presence of fly ash
1000 kHz than 580 kHz, because the very high frequency would cause adverse effects such small cavitation bubbles formed, extremely short lifetime, and too low collapse for sufficient sonodegradation of the IBP and SMX [40,41]. Previous studies have shown similar trends confirming an optimal frequency of 300 kHz among ultrasound frequencies tested from 192 to 960 kHz [42]; Guyer and Nince found that the highest removal of diclofenac was achieved at 861 kHz, compared with 577 and 1145 kHz [40]; Im et al. found the degradation of acetaminophen and naproxen was maximal at 580 kHz, in comparison with 28 and 1000 kHz [15]; US increased the degradation of IBP from 30 to 98% at 300 kHz at a contact time of 30 min [43]; the cavitation conditions in 20 kHz US limit the ability of IBP compared to those at 620 kHz to accumulate on bubble surfaces, which cause more competition with natural organic matter for bulk OH% at 20 kHz [44]. The results and explanation above were also confirmed by evaluating the generation of H2O2 at the three frequencies with IBP and SMX (Fig. 4). Maximum H2O2 production was obtained at 580 kHz, followed by 1000 and 28 kHz. This supports the effects of the frequency on the degradation of IBP and SMX. The H2O2 concentrations were higher with SMX than IBP due to their physicochemical properties (Table S1). As shown in Table S1, IBP has more hydrophobic and less soluble properties (log Kow 3.84 and 0.049 g L−1, respectively) than SMX (log Kow
The effect of the two types of fly ash, BFA and WFA, were investigated to estimate the enhancement of the sonodegradation of IBP and SMX under various ultrasound frequencies and pH conditions. Figs. 5 and 6 show the removal of IBP and SMX in the presence of BFA and WFA, respectively. As shown in both figures, the removal of IBP and SMX was enhanced significantly compared with the results in Fig. 3, which were obtained in the absence of fly ash. Several studies have linked sonochemical enhancement with the presence of different catalysts, including TiO2, CNTs, quartz, glass beads, and Al2O3 [5,18,47]. Previous research has shown that the presence of solid surfaces in the system promotes an increase in the number of cavitation bubbles, resulting in enhanced sonodegradation. This occurs because surface roughness has a tendency to increase the local temperature of cavitation bubbles, leading to increased water pyrolysis, thus, generating more OH%, and redistributing the US irradiation field. In turn, the cavitation active volume increases, thus, decreasing the threshold energy for the bubbles to collapse [5,11,48]. Generally, there are three reasons for the removal enhancement of IBP and SMX in the presence of fly ash: (i) sonochemical enhancement, due to the presence of solid surfaces that can increase the cavitational 358
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Fig. 5. Effect of ultrasonic frequency and pH on IBP and SMX removal in the presence of BFA (45 mg L−1); (a) IBP at pH 3.5, (b) IBP at pH 7, (c) IBP at pH 9.5, (d) SMX at pH 3.5, (e) SMX at pH 7, and (d) SMX at pH 9.5.
and regeneration of OH% radicals, which are responsible for the removal of IBP and SMX [51]. The adsorption enhancement from the adsorption activity of the fly ash with the IBP and SMX is assumed to be enhanced by the US process. It is known that US irradiation is one of the best methods to disperse and stabilize adsorbents, leading to an increase in adsorption sites and consequently enhanced adsorption of IBP and SMX [52]. Under US irradiation, the fly ash particles would be expected to be dispersed because the collapse of cavities creates high temperatures, causes pressure differences, and imparts shear forces on particle surfaces; thus, more adsorption sites are created [23,52]. Accordingly, the removal of IBP and SMX was higher in the presence of WFA than BFA in the adsorption reaction. This is because WFA had finer particles than BFA (Fig. 2), providing more adsorption sites, which could be the main reason for the difference in removal between them. As shown in Figs. 5 and 6, the removal of IBP and SMX in the presence of fly ash alone followed the order of pH 3.5 > pH 7 > pH 9.5 for both fly ashes. This is due to the physicochemical properties of IBP and SMX (Table 1S). The maximum hydrophobicity level was achieved when the pH value was lower than the pKa of the compounds [1], as previously discussed. Consequently, the removal of IBP in all reactions was higher than SMX, due to the differences in physicochemical properties; particularly, IBP has a higher hydrophobicity and lower solubility than SMX [5]. A higher hydrophobicity indicates higher
bubble production due to the surface roughness, (ii) due to the presence of metal oxides on the fly ash which can react with H2O2 to reproduce OH%, and (iii) adsorption enhancement, due to fly ash dispersion, which increases the adsorption sites. Sonochemical improvement may occur due to the following: the presence of fly ash particles, which increases the number of cavitation bubbles, and bubble nucleation due to entrapped gas or impurities on particle surfaces, surface reactivity of the particles, or the surface steadiness of the bubbles at the boundary; all could cause the collapse of cavitation bubbles [18,49,50]. Thus, the sonochemical degradation of IBP and SMX would be enhanced due to the presence of fly ash solid particles. Second, as shown in Table 1, the fly ash consists of many oxides that can react specifically with the H2O2 produced to regenerate the strong oxidant, OH%. As a result the degradation of IBP and SMX improves due to an increase in the oxidation process [18,49]. In this, catalytic decomposition is one of possible main reactions between H2O2 and oxides [51]. Catalytic decomposition is important to understand in terms of the reasons for enhanced sonodegradation. In catalytic decomposition, the H2O2 would be dissociated and form radical species that can bind to the surfaces where H2O2 undergoes decomposition; the radicals generated are stabilized by forming attachment states between the unpaired electron and the oxide surface [18,49,51]. The two main compounds in BFA and WFA are SiO2 and Al2O3; SiO2 and Al2O3 played major roles in this enhancement by dissociating the H2O2 produced via ultrasonication 359
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Fig. 6. Effect of ultrasonic frequency and pH on IBP and SMX removal in the presence of WFA (45 mg L−1); (a) IBP at pH 3.5, (b) IBP at pH 7, (c) IBP at pH 9.5, (d) SMX at pH 3.5, (e) SMX at pH 7, and (d) SMX at pH 9.5.
the removal of IBP and SMX. The synergistic indices of each process were determined from normalized differences between the rates constants obtained from the combined effects or process divided by the sum of the rate constants of the processes individually, as shown in Eqs. (5) and (6), adopted from [54]:
sorption affinity to the adsorbent (fly ash) [34]. Furthermore, it is also expected that the mineralization of PhACs may not be high because based on a previous study of similar pharmaceutical compounds at comparable experimental conditions (60 min contact time and 1000 kHz), less than 10% dissolved organic carbon removal was achieved for a mixture of acetaminophen and naproxen, which have similar physicochemical properties compared to IBP and SMX [14]. The results here were consistent with previous studies of sonocatalytical degradation using fly ash, such as that by Li et al. (2016), who reported significant enhancement of acid orange 7 degradation using fly ash and ultrasonication [13]. Li et al. (2015) compared the effects of different minerals in fly ash on the production of OH% radicals [53], and Hiroyuki et al. (2009) studied sonochemical and adsorption enhancement of hydrazine using coal ash [34].
Synergy index I =
Synergy index II =
k1 (US + BFA) k1 (US) + k1 (BFA) k1 (US + WFA) k1 (US) + k1 (WFA)
(5)
(6)
where k1 is the pseudo-first order reaction rate, ‘US’ indicates the US effect alone with no catalyst, ‘BFA’ the adsorption effect of BFA alone, with no US effect, and ‘WFA’ is the adsorption effect of WFA alone, with no US effect. Synergy index I evaluates the removal efficiency in the presence of BFA on the removal of IBP and SMX, while synergy index II evaluates the removal efficiency in the presence of WFA. A synergy index > 1 indicates that the combined reactions (i.e., US + BFW; US + WFA) process exceeds the sum of the individual reactions (i.e., US alone; BFA and WFA alone). As shown in Table 2, the synergy indices II (in the presence of WFA) were always greter than 1 and higher than those of synergy indices I (in the presence of BFA). This indicates that WFA had better ability to remove the selected pharmaceuticals compounds (IBP and SMX). As explained earlier, (i) WFA contains
3.4. Evaluation of synergistic indices of the fly ashes under different frequency and pH conditions The results of this study were summarized and analyzed by evaluating synergism in each reaction, to help in understanding the effects of frequency, pH, and fly ash as a catalyst under a US system. Table 2 provides a comprehensive evaluation and comparison on the effect of the three frequencies (28, 580, 1000 kHz), the three pH conditions (3.5, 7, 9.5), and the presence or absence of BFA and WFA on 360
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Table 2 Determination of the pseudo-first order (k1) removal rate constants, coefficient of determination (R2), and synergistic index values for all the reactions. Process
pH 3.5 IBP
SMX
Frequency 28 kHz
US only BFA w/US WFA w/US BFA w/o US WFA w/o US Synergy index I (BFA) Synergy index II (WFA)
R
k1
R
0.527 2.01 5.28 k1 0.604 1.23
0.943 0.997 0.996
4.37 5.81 10.3
0.977 0.995 0.999
2.28 4.10 8.34
0.945 0.990 0.998
R2 0.963 0.615 1.31 1.84 0.996 0.935 0.972
1.41 2.12
1.42 2.38
3.18 4.88 6.20
0.993 0.995 0.996
1.10 2.19 4.79
0.938 0.920 0.981
0.962 0.996 0.984
1.37 2.36
2.19 2.60 3.03
0.992 0.984 0.982
0.777 1.65 2.56
0.946 0.998 0.964
1.48 1.86
1000 kHz
k1
R
k1
R
k1
R2
0.360 0.814 2.01 k1 0.450 0.633
0.964 0.974 0.907
1.89 3.33 5.20
0.924 0.966 0.987 R2 0.985 0.941
1.44 2.13 3.59
0.903 0.947 0.981
0.218 0.546 1.23 k1 0.327 0.502
0.185 0.409 0.748 k1 0.249 0.335
2
1.42 2.06 0.906 0.935 0.927
1.01 1.71
R2 0.987 0.720 1.03 1.09
580 kHz 2
1.01 2.01
R2 0.984 0.685 1.33 1.51
pH 9.5 0.280 0.875 1.86 k1 0.340 0.599
BFA w/o US WFA w/o US Synergy index I (BFA) Synergy index II (WFA)
k1
1.27 2.29
US only BFA w/US WFA w/US
28 kHz 2
R
pH 7 0.299 1.01 2.81 k1 0.497 0.926
BFA w/o US WFA w/o US
1000 kHz 2
k1
1.78 3.01
US only BFA w/US WFA w/US
Synergy index I (BFA) Synergy index II (WFA)
580 kHz 2
1.46 2.42 3.96
1.13 1.73 0.958 0.987 0.979 R2 0.986 0.976
1.35 2.02 0.992 0.972 0.984
0.942 1.44
1.21 1.52 2.40
1.04 1.55
0.939 1.93 3.02
0.938 0.993 0.984
1.52 2.10 0.985 0.994 0.990 R2 0.970 0.985
0.595 1.04 1.39
0.970 0.986 0.999
1.23 1.49
more SiO2 (Table 1), which can enhance the generation of OH% radicals in the system, leading to increased oxidizing of IBP and SMX; and (ii) the particle size distribution (Fig. 2) showed that WFA had more particles with smaller sizes than BFA; therefore, WFA had a higher specific surface area and more adsorption sites than BFA, leading to an increase in the adsorption capacity for IBP/SMX on the WFA surface.
Acknowledgments
4. Conclusions
Appendix A. Supplementary data
In this study, the removal of IBP and SMX in the absence and presence of two fly ashes (BFA and WFA) was conducted at three frequencies (28, 580, 1000 kHz) and three pH values (3.5, 7, 9.5) with fixed conditions of power (0.18 W mL−1), temperature (15 °C), and contact time (60 min). The removal trends for IBP and SMX followed a pseudo first-order kinetic law in all reactions. The removal of IBP and SMX was enhanced significantly in the presence of fly ash under all conditions tested. The ‘best’ results for the removal of IBP and SMX were obtained at 580 kHz in the presence of WFA. The removal enhancement could be presumably achieved because (i) the presence of solid surfaces (fly ash) can increase the production of OH% radicals, increasing the reactions between OH% and IBP and SMX; (ii) the presence of certain oxides such as SiO2, Al2O3, and Fe2O3 can enhance the oxidation process, because they can react with the H2O2 produced from the ultrasonication and regenerate OH% radicals, leading to increased oxidation activity; and (iii) ultrasonic irradiation can disperse the fly ash, which reduces the effective particle size and leads to an increase in the surface area of the fly ash, enhancing adsorption activity.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2017.05.003.
This research was supported by a Grant (code 17IFIP-B088091-04) from Industrial Facilities & Infrastructure Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government. The authors also thank Holcim Inc. (Mr. Eddie Deaver) for their donation of fly ash samples.
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Sonocatalytic removal of ibuprofen and sulfamethoxazole in the presence of different fly ash sources
MARK
Yasir A.J. Al-Hamadania, Chang Min Parka, Lateef N. Assia, Kyoung Hoon Chua, Shamia Hoquea, ⁎ Min Jangb, Yeomin Yoona, , Paul Ziehla a b
Department of Civil and Environmental Engineering, University of South Carolina, Columbia, SC 29208, USA Department of Environmental Engineering, Kwangwoon University, 20 Kwangwoon-ro, Nowon-Gu, Seoul 01897, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Ibuprofen Sulfamethoxazole Sonocatalytical degradation Fly ash Water treatment
We examined the feasibility of using two types of fly ash (an industrial waste from thermal power plants) as a low-cost catalyst to enhance the ultrasonic (US) degradation of ibuprofen (IBP) and sulfamethoxazole (SMX). Two fly ashes, Belews Creek fly ash (BFA), from a power station in North Carolina, and Wateree Station fly ash (WFA), from a power station in South Carolina, were used. The results showed that > 99% removal of IBP and SMX was achieved within 30 and 60 min of sonication, respectively, at 580 kHz and pH 3.5. Furthermore, the removal of IBP and SMX achieved, in terms of frequency, was in the order 580 kHz > 1000 kHz > 28 kHz, and in terms of pH, was in the order of pH 3.5 > pH 7 > pH 9.5. WFA showed significant enhancement in the removal of IBP and SMX, which reached > 99% removal within 20 and 50 min, respectively, at 580 kHz and pH 3.5. This was presumably because WFA contains more silicon dioxide than BFA, which can enhance the formation of OH% radicals during sonication. Additionally, WFA has finer particles than BFA, which can increase the adsorption capacity in removing IBP and SMX. The sonocatalytic degradation of IBP and SMX fitted pseudo first-order rate kinetics and the synergistic indices of all the reactions were determined to compare the efficiency of the fly ashes. Overall, the findings have showed that WFA combined with US has potential for treating organic pollutants, such as IBP and SMX, in water and wastewater.
1. Introduction Over the last two decades, large quantities of products, such as medicines, disinfectants, and personal care products, have been released into surface waters and wastewater treatment facilities by the pharmaceutical and chemical industries [1]. Increases in the concentrations of some pharmaceutical compounds, such as ibuprofen (IBP) and sulfamethoxazole (SMX), have come to the attention of scientists with regard to their impacts on life in lakes, rivers, and groundwater [2,3]. Irregular disposal of unused medications, expired drugs, and veterinary medicines are the majors reasons why they end up in water bodies [4,5]. Concentrations of IBP and SMX in surface waters have been detected in the range of 30–480 ng L−1 [6,7], creating unique challenges, as conventional water treatment processes, including coagulation/sedimentation/filtration, typically can only remove 10–20% of these compounds [8,9]. Consequently, efforts are needed to find effective processes to remove these contaminants from water to meet the important goal of providing safe drinking water. Ultrasonic (US) treatment is one of the promising advanced oxida-
⁎
Corresponding author. E-mail address: [email protected] (Y. Yoon).
http://dx.doi.org/10.1016/j.ultsonch.2017.05.003 Received 4 January 2017; Received in revised form 18 April 2017; Accepted 2 May 2017 Available online 03 May 2017 1350-4177/ © 2017 Elsevier B.V. All rights reserved.
tion processes that has the potential to produce hydroxyl radicals (OH%) in water, which are strong oxidizing agents [10]. The power of OH% in water treatment lies in their ability to destroy and degrade complex, otherwise-hard-to-degrade, and toxic organic compounds and convert them ultimately to carbon dioxide and water [11]. The process of US treatment produces OH% through the cavitation phenomenon and the formation of high-intensity bubbles [12]. Cavitation occurs very quickly, through the steps of nucleation, growth, and the collapse of cavitation bubbles in water, releasing large amounts of energy locally, generating hot spots, and producing hydrogen and OH% due to the sonolysis of water [5,10]. During this phenomenon, high temperatures (5000 K) and pressures (1000 atm) created inside cavitation bubbles lead to thermal dissociation of water molecules into H% and OH% [13]. The O2 dissolved in water reacts and forms OH% and HO2%. Additionally, the cavitation bubbles contain three zones: the gaseous zone, the gas–liquid transition zone, and the bulk liquid zone. In the gaseous zone, the temperature and pressure reach their maximum levels of 5000 K and 1000 atm, respectively. The zone is hydrophobic and volatile compounds can be degraded. Second, in the gas–liquid
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and efficient in producing a consistent, high-quality fly ash [26]. Stock solutions of IBP, SMX, BFA, and WFA were prepared in ultrapure deionized (DI) water.
transition zone, the temperature reaches 2000 K. The zone is moderately hydrophobic and moderate degradation of volatile compounds can be achieved. The third zone is the bulk liquid zone, where the temperature is 300 K; hydrophilic and non-volatile compounds undergo degradation in this zone [14–16]. Previous studies have indicated that US treatment has marked benefits, including safety, cleanliness, and ease of use; additionally, no carcinogenic by-products form during treatment [5,11]. Many studies have reported that sonochemical degradation can be enhanced by the presence of solid surfaces as catalysts, such as TiO2, quartz, glass beads, polyaluminum chloride, Al2O3, and carbon nanotubes [5,15,17,18]. Because US treatment is highly energy-intensive, catalysts are needed to improve the removal efficiency and reduce the effective energy consumption [17]. However, such catalysts are relatively expensive for treating large volumes of wastewater. Thus, alternative low-cost catalysts need to be investigated for their ability to remove contaminants, such as pharmaceutical compounds. Fly ash was assessed in this study regarding its ability to enhance the sonodegradation of IBP and SMX. Fly ash is a by-product waste material generated in dry form in thermal power plants. Large amounts of fly ash are generated and dumped in landfills annually [19–21]. Thus, using fly ash in water and wastewater treatment is a good strategy to reduce environmental pollution. Fly ash’s chemical composition generally consists of aluminum oxide (Al2O3) and silicon dioxide (SiO2) (in total, 60–80 wt%), in addition to some transition metal oxides [13,19]. Previous studies have found fly ash to be a good adsorbent for various types of dyes [22,23], and it has been used in photocatalytic applications, combined with TiO2 [24]. However, only few studies have investigated the use of fly ash as a catalyst under ultrasonic irradiation, despite its ability to enhance significantly the sonodegradation of acid orange 7 [13]. This suggests that fly ash may have the potential to enhance the removal of pharmaceutical compounds, such as IBP and SMX, under different frequency and pH conditions. Thus, the objectives of this study were to evaluate the removal of IBP and SMX at different US frequency and pH conditions in the presence and absence of fly ash. Reactions were carried out as a function of frequency (28, 580, and 1000 kHz) and pH (3.5, 7, and 9.5) at a fixed power (0.18 W mL−1) and room temperature (15 °C). The contribution of this work was to investigate an alternative low-cost catalyst (fly ash) that may enhance the removal of IBP and SMX. Two hypotheses were tested. First, fly ash should enhance the removal of IBP and SMX, due to the increased production of OH% radicals (a strong oxidant). This is presumably because fly ash contains sufficient amounts of Al2O3 and SiO2 that can react with the hydrogen peroxide and generate OH% radicals. Second, US irradiation should enhance the adsorption activities of fly ash due to the dispersion resulting from the harsh conditions provided by US irradiation. This would be expected to lead to an increase in the adsorption sites on fly ash particles.
2.2. Apparatus The US process was performed in a double-jacketed stainless steel reactor (L × W × H: 15 × 10 × 20 cm) with a water-cooled (Fisher Scientific Inc., Pittsburgh, PA, USA) US generator (Ultech, Dalseo, Daegu, South Korea). The sonicator provided three test frequencies: 28, 580, and 1000 kHz. The applied power in all tests was 0.18 W mL−1. The US applied power was measured by calorimetry by thermocouple (HI 9063, Hanna Instruments Ltd., Leighton, UK). Fig. S1 shows a diagram of the experimental set-up. Because BFA and WFA were used as catalysts, an optimum dose was determined based on H2O2 production at different fly ash dosages and frequencies. Adsorption experiments with the adsorbents (BFA and WFA) and adsorbates (IBP and SMX) were performed for 60 min, in a batch reactor with no US irradiation. BFA and WFA were hydrated for 24 h in DI water and stirred with a magnetic stirrer at 600 rpm prior to being added to the reactor vessel. The initial stock solution was 1000 mL, which was used in all experiments (US only, US with BFA/WFA, and the adsorption experiments). Samples were taken periodically and filtered through 0.22-μm glass microfiber filters to preserve uniformity and to eliminate larger fly ash particles that might interfere with the measurements. 2.3. Analysis IBP and SMX concentrations before and after treatment were measured using high-performance liquid chromatography (HPLC, 1200 series; Agilent Technologies, Santa Clara, CA, USA). The mobile phase was a 40%:60% mixture of deionized water:acetonitrile for IBP and a 50%:50% deionized water:acetonitrile for SMX. Separation was achieved with a LiChrosorb RP-18 analytical column (4.6 mm × 100 mm i.d., 5 µm particles, Atlantis; Waters, Milford, MA, USA) with a 100-µL sample loop at a flow rate of 1.0 mL min−1 for IBP and 0.75 mL min−1 for SMX. The wavelength used to detect the compounds was 210 nm. The KI dosimetry method was used to determine the H2O2 concentration, as an indicator of OH% free radicals in the system [27], using a 350-nm wavelength and an ultraviolet–visible (UV–Vis) spectrophotometer (Agilent Technologies, Santa Clara, CA, USA). The elemental composition of BFA and WFA fly ash was determined by X-ray florescence (XRF) using fused bead analysis at the Holcim Inc. laboratory in Holly Hill, SC, USA. 3. Results and discussion 3.1. Dose optimization and characterization of BFA and WFA Determining the optimum dose of fly ash is important in comparing the efficiency of BFA and WFA under US irradiation. The optimum dose was determined based on the H2O2 produced under irradiation; as a rule, the amount of H2O2 increases with the amount of OH% radicals. Thus, H2O2 was measured as an indicator of OH% radicals in the system [28]. Previous studies showed that the presence of solid surfaces can improve the sonodegradation of contaminants by increasing the formation rate of cavitation bubbles; the presence of solid particle in solution provides a nucleation site due to surface roughness, leading to increased generation of OH% radicals in the system [5,29]. The optimum values of various doses (0, 5, 15, and 45 mgL−1) of BFA and WFA were investigated at three frequencies (28, 580, and 1000 kHz) at pH 7, as shown in Fig. 1. The results clearly indicated that the maximum production rate of H2O2 was achieved at 45 mg L−1 at all frequencies for both BFA and WFA, However, WFA showed higher H2O2 production than BFA due to differences in the chemical properties between the fly ashes.
2. Materials and methods 2.1. Chemicals and fly ashes Table S1 lists the characteristics of the selected target chemicals (IBP and SMX) from the SRC PhysProp database [25]. All chemicals were used as received with no further purification. High-purity IBP (C13H18O2, > 98%), SMX (C10H11N3O3S, > 98%), potassium hydrogen phthalate (C8H5KO4, 99.95%), potassium iodide (KI, 99%), and ammonium molybdate tetrahydrate (H24Mo7N6O24·4H2O) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Two fly ashes, Belews Creek fly ash (BFA) from a power station in North Carolina and Wateree Station fly ash (WFA) from a power station in South Carolina, were investigated. The main difference between the two sources is that the Wateree Station source was subjected to a proprietary carbon burn-out process. The carbon burn-out process has been shown to be effective 355
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Fig. 1. Effect of ultrasonic frequency on H2O2 production in the presence of BFA and WFA; (a) BFA at 28 kHz, (b) BFA at 580 kHz, (c) BFA at 1000 kHz, (d) WFA at 28 kHz, (e) WFA at 580 kHz, and (f) WFA at 1000 kHz at pH 7.
leading to enhanced oxidation activity as described in Eqs. (13) [34]:
Table 1 XRF chemical composition analysis of BFA and WFA. Compound
Chemical formula
BFA wt.%
WFA wt.%
Silicon dioxide Aluminum oxide Iron oxide Calcium oxide Magnesium oxide Sulfur trioxide Sodium dioxide Potassium oxide Total alkali Diphosphorus pentoxide Titanium dioxide Manganese oxide
SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2 O – P2O5 TiO2 MnO2
48.9 20.9 6.9 3.9 1.0 0.6 5.0 2.7 8.7 0.3 1.0 0.1
58.4 22.4 6.6 0.6 0.6 0.1 2.4 2.1 5.5 0.1 1.1 0.1
Fe 2+ + H2 O2 → Fe3+ + OH% + OH−
(1)
Fe3+ + H2 O2 → Fe−O2 H2+ + H+
(2)
Fe−O2 H2+ → Fe 2+ + OOH%
(3)
The decomposition of H2O2 in the presence of fly ash containing metal oxides can be described by the Haber-Weiss mechanism [35]. In the Harber-Weiss mechanism, the principal role of H2O2 is the oxidation of the metal surface, which leads to the formation of hydroxyl radicals as described in Eq. (4),
S + H2 O2 → S + + OH% + OH−
(4)
where S represents the uncharged metal surface [36]. Thus, Fe2O3 has the potential to produce more OH% radicals by reacting with the H2O2 formed, leading to enhanced sonodegradation of the compounds. The remaining constituents in the fly ashes were not considered due to their trace amounts and their presumably minor or negligible effects on sonodegradation. Taken together, our results showed that WFA contained more SiO2 than BFA (Table 1), which increased the production of OH% radicals. Because the fly ash consists of a wide range of particles sizes, it was important to define the particle size distributions of BFA and WFA (Fig. 2). WFA contained finer particles than BFA; the average particles size for BFA and WFA were 21.3 and 15.2 µm, respectively, indicating an average particle size difference of 29%. This could be responsible for the adsorption behavior of the fly ashes because smaller particles have
The elemental composition of BFA and WFA is summarized in Table 1. SiO2 was a major component in both fly ashes. SiO2 is also a main component of glass beads, which have been shown in previous studies to be effective in increasing the generation of OH% radicals in US processes [5,30,31]. The presence of Al2O3 can also increase OH% because (i) the oxide would bind OH% radicals and thereby decrease the formation of H2O2 and (ii) both SiO2 and Al2O3 can react with the H2O2 produced due to the sonication, and reproduce OH% radicals [32,33]. A third major constituent in the selected fly ashes was Fe2O3. Fe2O3 can also enhance the production of OH% radicals in an aqueous system by reaction with the H2O2 produced; this in turn allows OH% to reform, 356
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decrease followed a pseudo first-order kinetic law, and the removal of IBP was the highest at 580 kHz (> 99% at 30 min, > 99% at 55 min, and 95% at 60 min at pH 3.5, 7, and 9.5, respectively) followed by 1000 kHz (98%, 77%, and 62% at pH 3.5, 7, and 9.5, respectively) at 60 min, and the significantly lower removal was obtained at 28 kHz (56%, 33%, and 22% at pH 3.5, 7, and 9.5, respectively) at 60 min. SMX removal showed a similar trend where maximum removal was obtained at 580 kHz (> 99%, 90%, and 76% at pH 3.5, 7, and 9.5, respectively) at 60 min, followed by 1000 kHz (92%, 70%, and 47% at pH 3.5, 7, and 9.5, respectively) and 28 kHz (43%, 21%, and 19% at pH 3.5, 7, and 9.5, respectively) at 60 min. The main factors at the different frequencies were the number of cavitation bubbles, bubble size, the cavitation threshold, and the lifetime of the bubbles before collapsing [5,37]. At 28 kHz, fewer cavitation bubbles were produced, the growth period to collapse was high, and large bubbles formed [38]; thus, fewer OH% free radicals were produced, leading to less degradation of IBP and SMX, due to the reduction in the oxidation activity in the system. In contrast, at higher frequencies (580 and 1000 kHz) the number of cavitation bubbles increased and the lifetime of the bubbles to collapse decreased. Overall, the more number of cavitation bubbles lead to increase the numbers of OH% free radicals. Thus the chances of the recombining and forming H2O2 might be higher as well. However, at high frequency, the retarding effect and the free radicals tended to move quickly towards the bulk liquid [15,38], reducing the possibility of OH% recombination and formation of H2O2; this resulted in increased removal of IBP and SMX [39]. However, a lower degradation rate was achieved at
Fig. 2. Particle size distribution of BFA and WFA.
more surface area and adsorption capacity for IBP and SMX. 3.2. Effects of frequency and pH on IBP and SXM removal and H2O2 production In the sonodegradation process, there are two important factors, frequency and pH, along with the physicochemical properties of the compounds, which play major roles in the degradation of contaminants. Fig. 3 shows the degradation of IBP and SMX at the three frequencies and three pH conditions (pH 3.5, 7, and 9.5). The concentration
Fig. 3. Effect of ultrasonic frequency and pH on IBP and SMX removal; (a) IBP at pH 3.5, (b) IBP at pH 7,(c) IBP at pH 9.5, (d) SMX at pH 3.5, (e) SMX at pH 7, and (d) SMX at pH 9.5.
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0.79 and 0.5 g L−1, respectively) as well as different pKa values (4.52 and 5.81 for IBP and SMX, respectively), which have combined effects on sonodegradation activity. Therefore, IBP had a tendency to consume more OH% radicals than SMX from the system, leading to more degradation of IBP than SMX, resulting in less recombination of OH% radicals to form H2O2 [39]. The second important factor influencing sonodegradation is the pH. Three pH values (3.5, 7, and 9.5) were investigated with the three frequencies to better understand the sonodegradation of IBP and SMX at a fixed power (0.18 W mL−1). The removal of both IBP and SMX was highest under acidic conditions, pH 3.5, and it decreased at pH 7 and pH 9.5 (Fig. 3). The degradation of IBP and SMX decreased as pH increased because the physicochemical properties of the compounds would put them in their molecular forms when the pH was lower than the pKa values, while above the pKa values, they would be in ionic forms [14]. The higher degradation of IBP and SMX under acidic conditions and US is because IBP and SMX, in their molecular forms, tend to accumulate at the boundary of the cavitation bubbles where the concentration of OH% radicals is maximal. In contrast, in their ionic forms, when the pH is higher than their pKa values, the compounds tend to move towards the bulk zone where the OH% concentration is lower [45,46]. Thus, under acidic conditions, more degradation of IBP and SMX was achieved because the reaction between the OH% radicals and the IBP/SMX occurred in a zone with a higher density of OH% radicals at the boundary of the cavitation bubbles and less under alkaline conditions because the reaction occurred in the bulk liquid zone, where fewer OH% radicals are found. The production of H2O2 was assessed at the selected pH values. Clearly, Fig. 4 shows that the generation of H2O2 followed a trend opposite to that of the degradation of IBP and SMX, in which the maximum H2O2 was produced at pH 9.5, followed by pH 7 and pH 3.5. This is because at low pH, the OH% free radicals have a tendency to attach and react with IBP and SMX more than recombining to produce H2O2, and thus more degradation and less H2O2 were achieved than at higher pH conditions [14,39]. In addition, in the absence of IBP and SMX, the production of H2O2 followed the order of 580 kHz > 1000 kHz > 28 kHz and pH 9.5 > pH 7 > pH 3.5 as shown in Fig. S2. The maximum H2O2 generated was achieved at 580 kHz (170 µM at pH 3.5, 211 µM at pH 7, and 232 µM at pH 9.5) followed by 1000 kHz (63 µM at pH 3.5, 81 µM at pH 7, and 105 µM at pH 9.5) and lowest H2O2 production was obtained at 28 kHz (12 µM at pH 3.5, 17 µM at pH 7, and 21 µM at pH 9.5).
Fig. 4. Effect of ultrasonic frequency and pH on H2O2 production in the presence of IBP and SMX at (a) pH 3.5, (b) pH 7, and (c) pH 9.5.
3.3. Effects of frequency and pH on IBP and SMX removal in the presence of fly ash
1000 kHz than 580 kHz, because the very high frequency would cause adverse effects such small cavitation bubbles formed, extremely short lifetime, and too low collapse for sufficient sonodegradation of the IBP and SMX [40,41]. Previous studies have shown similar trends confirming an optimal frequency of 300 kHz among ultrasound frequencies tested from 192 to 960 kHz [42]; Guyer and Nince found that the highest removal of diclofenac was achieved at 861 kHz, compared with 577 and 1145 kHz [40]; Im et al. found the degradation of acetaminophen and naproxen was maximal at 580 kHz, in comparison with 28 and 1000 kHz [15]; US increased the degradation of IBP from 30 to 98% at 300 kHz at a contact time of 30 min [43]; the cavitation conditions in 20 kHz US limit the ability of IBP compared to those at 620 kHz to accumulate on bubble surfaces, which cause more competition with natural organic matter for bulk OH% at 20 kHz [44]. The results and explanation above were also confirmed by evaluating the generation of H2O2 at the three frequencies with IBP and SMX (Fig. 4). Maximum H2O2 production was obtained at 580 kHz, followed by 1000 and 28 kHz. This supports the effects of the frequency on the degradation of IBP and SMX. The H2O2 concentrations were higher with SMX than IBP due to their physicochemical properties (Table S1). As shown in Table S1, IBP has more hydrophobic and less soluble properties (log Kow 3.84 and 0.049 g L−1, respectively) than SMX (log Kow
The effect of the two types of fly ash, BFA and WFA, were investigated to estimate the enhancement of the sonodegradation of IBP and SMX under various ultrasound frequencies and pH conditions. Figs. 5 and 6 show the removal of IBP and SMX in the presence of BFA and WFA, respectively. As shown in both figures, the removal of IBP and SMX was enhanced significantly compared with the results in Fig. 3, which were obtained in the absence of fly ash. Several studies have linked sonochemical enhancement with the presence of different catalysts, including TiO2, CNTs, quartz, glass beads, and Al2O3 [5,18,47]. Previous research has shown that the presence of solid surfaces in the system promotes an increase in the number of cavitation bubbles, resulting in enhanced sonodegradation. This occurs because surface roughness has a tendency to increase the local temperature of cavitation bubbles, leading to increased water pyrolysis, thus, generating more OH%, and redistributing the US irradiation field. In turn, the cavitation active volume increases, thus, decreasing the threshold energy for the bubbles to collapse [5,11,48]. Generally, there are three reasons for the removal enhancement of IBP and SMX in the presence of fly ash: (i) sonochemical enhancement, due to the presence of solid surfaces that can increase the cavitational 358
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Fig. 5. Effect of ultrasonic frequency and pH on IBP and SMX removal in the presence of BFA (45 mg L−1); (a) IBP at pH 3.5, (b) IBP at pH 7, (c) IBP at pH 9.5, (d) SMX at pH 3.5, (e) SMX at pH 7, and (d) SMX at pH 9.5.
and regeneration of OH% radicals, which are responsible for the removal of IBP and SMX [51]. The adsorption enhancement from the adsorption activity of the fly ash with the IBP and SMX is assumed to be enhanced by the US process. It is known that US irradiation is one of the best methods to disperse and stabilize adsorbents, leading to an increase in adsorption sites and consequently enhanced adsorption of IBP and SMX [52]. Under US irradiation, the fly ash particles would be expected to be dispersed because the collapse of cavities creates high temperatures, causes pressure differences, and imparts shear forces on particle surfaces; thus, more adsorption sites are created [23,52]. Accordingly, the removal of IBP and SMX was higher in the presence of WFA than BFA in the adsorption reaction. This is because WFA had finer particles than BFA (Fig. 2), providing more adsorption sites, which could be the main reason for the difference in removal between them. As shown in Figs. 5 and 6, the removal of IBP and SMX in the presence of fly ash alone followed the order of pH 3.5 > pH 7 > pH 9.5 for both fly ashes. This is due to the physicochemical properties of IBP and SMX (Table 1S). The maximum hydrophobicity level was achieved when the pH value was lower than the pKa of the compounds [1], as previously discussed. Consequently, the removal of IBP in all reactions was higher than SMX, due to the differences in physicochemical properties; particularly, IBP has a higher hydrophobicity and lower solubility than SMX [5]. A higher hydrophobicity indicates higher
bubble production due to the surface roughness, (ii) due to the presence of metal oxides on the fly ash which can react with H2O2 to reproduce OH%, and (iii) adsorption enhancement, due to fly ash dispersion, which increases the adsorption sites. Sonochemical improvement may occur due to the following: the presence of fly ash particles, which increases the number of cavitation bubbles, and bubble nucleation due to entrapped gas or impurities on particle surfaces, surface reactivity of the particles, or the surface steadiness of the bubbles at the boundary; all could cause the collapse of cavitation bubbles [18,49,50]. Thus, the sonochemical degradation of IBP and SMX would be enhanced due to the presence of fly ash solid particles. Second, as shown in Table 1, the fly ash consists of many oxides that can react specifically with the H2O2 produced to regenerate the strong oxidant, OH%. As a result the degradation of IBP and SMX improves due to an increase in the oxidation process [18,49]. In this, catalytic decomposition is one of possible main reactions between H2O2 and oxides [51]. Catalytic decomposition is important to understand in terms of the reasons for enhanced sonodegradation. In catalytic decomposition, the H2O2 would be dissociated and form radical species that can bind to the surfaces where H2O2 undergoes decomposition; the radicals generated are stabilized by forming attachment states between the unpaired electron and the oxide surface [18,49,51]. The two main compounds in BFA and WFA are SiO2 and Al2O3; SiO2 and Al2O3 played major roles in this enhancement by dissociating the H2O2 produced via ultrasonication 359
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Fig. 6. Effect of ultrasonic frequency and pH on IBP and SMX removal in the presence of WFA (45 mg L−1); (a) IBP at pH 3.5, (b) IBP at pH 7, (c) IBP at pH 9.5, (d) SMX at pH 3.5, (e) SMX at pH 7, and (d) SMX at pH 9.5.
the removal of IBP and SMX. The synergistic indices of each process were determined from normalized differences between the rates constants obtained from the combined effects or process divided by the sum of the rate constants of the processes individually, as shown in Eqs. (5) and (6), adopted from [54]:
sorption affinity to the adsorbent (fly ash) [34]. Furthermore, it is also expected that the mineralization of PhACs may not be high because based on a previous study of similar pharmaceutical compounds at comparable experimental conditions (60 min contact time and 1000 kHz), less than 10% dissolved organic carbon removal was achieved for a mixture of acetaminophen and naproxen, which have similar physicochemical properties compared to IBP and SMX [14]. The results here were consistent with previous studies of sonocatalytical degradation using fly ash, such as that by Li et al. (2016), who reported significant enhancement of acid orange 7 degradation using fly ash and ultrasonication [13]. Li et al. (2015) compared the effects of different minerals in fly ash on the production of OH% radicals [53], and Hiroyuki et al. (2009) studied sonochemical and adsorption enhancement of hydrazine using coal ash [34].
Synergy index I =
Synergy index II =
k1 (US + BFA) k1 (US) + k1 (BFA) k1 (US + WFA) k1 (US) + k1 (WFA)
(5)
(6)
where k1 is the pseudo-first order reaction rate, ‘US’ indicates the US effect alone with no catalyst, ‘BFA’ the adsorption effect of BFA alone, with no US effect, and ‘WFA’ is the adsorption effect of WFA alone, with no US effect. Synergy index I evaluates the removal efficiency in the presence of BFA on the removal of IBP and SMX, while synergy index II evaluates the removal efficiency in the presence of WFA. A synergy index > 1 indicates that the combined reactions (i.e., US + BFW; US + WFA) process exceeds the sum of the individual reactions (i.e., US alone; BFA and WFA alone). As shown in Table 2, the synergy indices II (in the presence of WFA) were always greter than 1 and higher than those of synergy indices I (in the presence of BFA). This indicates that WFA had better ability to remove the selected pharmaceuticals compounds (IBP and SMX). As explained earlier, (i) WFA contains
3.4. Evaluation of synergistic indices of the fly ashes under different frequency and pH conditions The results of this study were summarized and analyzed by evaluating synergism in each reaction, to help in understanding the effects of frequency, pH, and fly ash as a catalyst under a US system. Table 2 provides a comprehensive evaluation and comparison on the effect of the three frequencies (28, 580, 1000 kHz), the three pH conditions (3.5, 7, 9.5), and the presence or absence of BFA and WFA on 360
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Table 2 Determination of the pseudo-first order (k1) removal rate constants, coefficient of determination (R2), and synergistic index values for all the reactions. Process
pH 3.5 IBP
SMX
Frequency 28 kHz
US only BFA w/US WFA w/US BFA w/o US WFA w/o US Synergy index I (BFA) Synergy index II (WFA)
R
k1
R
0.527 2.01 5.28 k1 0.604 1.23
0.943 0.997 0.996
4.37 5.81 10.3
0.977 0.995 0.999
2.28 4.10 8.34
0.945 0.990 0.998
R2 0.963 0.615 1.31 1.84 0.996 0.935 0.972
1.41 2.12
1.42 2.38
3.18 4.88 6.20
0.993 0.995 0.996
1.10 2.19 4.79
0.938 0.920 0.981
0.962 0.996 0.984
1.37 2.36
2.19 2.60 3.03
0.992 0.984 0.982
0.777 1.65 2.56
0.946 0.998 0.964
1.48 1.86
1000 kHz
k1
R
k1
R
k1
R2
0.360 0.814 2.01 k1 0.450 0.633
0.964 0.974 0.907
1.89 3.33 5.20
0.924 0.966 0.987 R2 0.985 0.941
1.44 2.13 3.59
0.903 0.947 0.981
0.218 0.546 1.23 k1 0.327 0.502
0.185 0.409 0.748 k1 0.249 0.335
2
1.42 2.06 0.906 0.935 0.927
1.01 1.71
R2 0.987 0.720 1.03 1.09
580 kHz 2
1.01 2.01
R2 0.984 0.685 1.33 1.51
pH 9.5 0.280 0.875 1.86 k1 0.340 0.599
BFA w/o US WFA w/o US Synergy index I (BFA) Synergy index II (WFA)
k1
1.27 2.29
US only BFA w/US WFA w/US
28 kHz 2
R
pH 7 0.299 1.01 2.81 k1 0.497 0.926
BFA w/o US WFA w/o US
1000 kHz 2
k1
1.78 3.01
US only BFA w/US WFA w/US
Synergy index I (BFA) Synergy index II (WFA)
580 kHz 2
1.46 2.42 3.96
1.13 1.73 0.958 0.987 0.979 R2 0.986 0.976
1.35 2.02 0.992 0.972 0.984
0.942 1.44
1.21 1.52 2.40
1.04 1.55
0.939 1.93 3.02
0.938 0.993 0.984
1.52 2.10 0.985 0.994 0.990 R2 0.970 0.985
0.595 1.04 1.39
0.970 0.986 0.999
1.23 1.49
more SiO2 (Table 1), which can enhance the generation of OH% radicals in the system, leading to increased oxidizing of IBP and SMX; and (ii) the particle size distribution (Fig. 2) showed that WFA had more particles with smaller sizes than BFA; therefore, WFA had a higher specific surface area and more adsorption sites than BFA, leading to an increase in the adsorption capacity for IBP/SMX on the WFA surface.
Acknowledgments
4. Conclusions
Appendix A. Supplementary data
In this study, the removal of IBP and SMX in the absence and presence of two fly ashes (BFA and WFA) was conducted at three frequencies (28, 580, 1000 kHz) and three pH values (3.5, 7, 9.5) with fixed conditions of power (0.18 W mL−1), temperature (15 °C), and contact time (60 min). The removal trends for IBP and SMX followed a pseudo first-order kinetic law in all reactions. The removal of IBP and SMX was enhanced significantly in the presence of fly ash under all conditions tested. The ‘best’ results for the removal of IBP and SMX were obtained at 580 kHz in the presence of WFA. The removal enhancement could be presumably achieved because (i) the presence of solid surfaces (fly ash) can increase the production of OH% radicals, increasing the reactions between OH% and IBP and SMX; (ii) the presence of certain oxides such as SiO2, Al2O3, and Fe2O3 can enhance the oxidation process, because they can react with the H2O2 produced from the ultrasonication and regenerate OH% radicals, leading to increased oxidation activity; and (iii) ultrasonic irradiation can disperse the fly ash, which reduces the effective particle size and leads to an increase in the surface area of the fly ash, enhancing adsorption activity.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ultsonch.2017.05.003.
This research was supported by a Grant (code 17IFIP-B088091-04) from Industrial Facilities & Infrastructure Research Program funded by Ministry of Land, Infrastructure and Transport of Korean government. The authors also thank Holcim Inc. (Mr. Eddie Deaver) for their donation of fly ash samples.
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